Electromagnetic (EM) geophysical surveying involves imparting an electric field or a magnetic field into subsurface Earth formations, such formations being below a body of water such as a sea, river, lake, or ocean in marine surveys, and measuring electric field amplitude and/or amplitude of magnetic fields by measuring voltage differences induced in electrodes, antennas and/or interrogating magnetometers disposed at the Earth's surface, or on or above the floor of the body of water. The electric and/or magnetic fields are induced in response to the electric field and/or magnetic field imparted into the Earth's subsurface, and inferences about the spatial distribution of conductivity of the Earth's subsurface are made from recordings of the induced electric and/or magnetic fields.
Electromagnetic surveying may also involve imparting a time-varying electromagnetic field into the subsurface formations by passing time-varying electric current through a transmitter antenna. The alternating current may have one or more selected discrete frequencies. Such surveying is known as frequency-domain controlled-source electromagnetic (f-CSEM) surveying. Another technique is known as transient controlled-source electromagnetic surveying (t-CSEM). In t-CSEM, electric current is passed through a transmitter at the Earth's surface (or near the floor of a body of water), in a manner similar to f-CSEM. The electric current may be direct current (DC). At a selected time, the electric current is switched off, switched on, or has its polarity changed, and induced voltages and/or magnetic fields are measured, typically with respect to time over a selected time interval, at the Earth's surface or water surface. Alternative switching techniques are possible.
The above methods for f-CSEM and t-CSEM have been adapted for use in marine environments. Cable-based sensors have been devised for detecting electric and/or magnetic field signals resulting from imparting electric and/or magnetic fields into formations below the bottom of a body of water. Systems with towed electromagnetic receivers have also been devised.
Note that the figures provided herewith are not necessarily to scale. They are provided for purposes of illustration to ease in the understanding of the presently-disclosed invention.